Photographic emulsions peptized with zein



Patented Aug. 7, 1951 2,563,791 euorocasrmcmmnsrons PERTIZED I'M'IIHZEIN Wesley G. Lowe, Rochester; N. Y., 'assignor to EastmanKodakrGnmpa corporation*ofiNewJersey "No Drawing.Application'scptonibet'fl 7 Serial-No. 4737-53 a a Qlaims. (ClnBfi-m?)This invention relates to "the preparation of photographic emulsionscharacterized' by the of zeinas the peptizirlg agentfor the silverhalide employed therein.

:Inthe preparation of silver halide type photographic emulsions it-isdesirable to prepare-the silver ghalide in a finely divided form;accomplished ordinarily "by a double decomposition reaction in anaqueous solution .of a "colloid (known as a peptizer). Gelatin-msbeenfound to be themost useful for {this purpose but other colloidalsubstances have "been mentioned as being suitable. When dispersions,ofisilver halide are desired having a different susceptibility to waterthan gelatinya problem arisesas mosttofthe water-insoluble colloidsdonothave good peptizing properties.

'Qne ,object of my invention is to provide a peptizer ior silver halidewhich is vwater'insoluble. Another object of my invention is ftopr'ovide a method vwherebymixed grain emulsions which are separately.developahle may be prepared. gOther vobjects will appear herein.

I have "found that zein when'in solution in a lower alcohol or acetoneand water acts as a peptizer for silver halide upon its jformationandthat the resulting dispersion maybe employed in the preparation ofphotographic emulsions. I

have found that zein-peptized dispersions of silver halide which aredifferentially developable from gelatin-peptized silver halide{dispersions may be prepared and employed in making photographicemulsions having unique characteristics. zein has characteristicswhichespeciady adapt it for mixed grain type remulsions; Some 413i thesecharacteristics are: 1. :Unlike the "majority of vva'i'er'einsolublec01- loids,-zein is-avery good p'eptizer of silver halide. 2. Unlikemany water-insoluble colloids it tolerates the presence "of enough'water in some of its solvent mixtures to make emulsion makingconvenient by 'conventional methods. 3. Zein is soluble in benzylalcohol, which solvent has characteristics making it 'a good medium fordispersion in "gelatin solutions. Some Of these characteristics ofbenzyl alcohcil are:

a. 'Benzyl alcohol is a good solvent for zein, for many sensitizing dyesandrmany couplers. i). It is sufficiently --water-insoluble *to makeconvenient formation of a fine-grained, uniform dispersion in gelatinsolutions by means-of a colloidmill. Y i c, It is soluble enoug'hinwater to be remolied :2 from a-photog-raphic iemulsion 'by watenwashinthei emulsion whlle ininoodle rorm.

1 d. ,Benzyl alcohol *lsva readily available nonfog ging :solverit.

4. Zein-peptized emulsions are satisfaotory in the fineness erdispersion of the silver "halide, photographic stability and ireedom'from "fog.

5; Drdinarydevelopersgnd"fixing'baths can be used for processing theemulsion, audit develops readily in conplerdevelopers.

6."The envelope -or zein "around the silver halide :graii'ns acts as aba irrier-to the passage of sensitizing dyes, so 'that contamination ofneighboring qgrains 'by transfer oi the optical-sensitizer can' -be asolution of'rein in *isopropyl alcohol-water is particularly useful asfapeptizer for silver halide. 'lI-he silver halide dispersion. may beprepared "by introducing a stream of silver nitrate solution and astream of an alkali metal halide :solution into the zein solutionsimultaneously wit'h stirring or byladding the alkali metal halide ftpthe solution ofaein and introducing a stream of silver nitrate solution:withstirring. Thereresultsa'nnely divided silver, halide in suspensionin the zein "solution. "This dispersion may then be'incorporatedinasolution of acompatlble "emulsion" vehicle or preferably is first washedand dispersed in benzyl alcohol, which dispersion may then be dispersedin gelatin or some other vehicle by means of a colloid mill or similardispersing "mechanism. Such a zein emulsion dispersed .in :geflatin canbe developed and 11nd idemirinfll gpr'ocessing solutions.

It, howeverpanpmul'sion is desired which is deveiop'able ata-=pH,greater than 9, the zein emu s ndispsrsed se s may be gelled andhandled and thernoodles washed with water to "remove-the benzyl alcohol.The resulting emulsionisicomposed of small particles of :ze-in enclosing:silver vfhalide crystals and dispersed in :gelatin. ilt' is :less'developahle-at ra' low pH than a similar emulsion prepared in gelatinand .polyviny l .alcohol. M .a rhigh cpl-I, such as above *9 and up to12, it is readily developed. 'These emulsions are diflicult to sensitizeii the sensitizer is added to the final emulsion due to the zein barriersurrounding-"the grains, *butthey are readily sensitized if the'sensitizer is added *to the benzyl alcohol solution before dispersingit in gelatin.

To prepare amixed grainyphotographic emulsion the zein-peptiaed emulsionas described herein y be mixed -wlth the conventional type high pHdeveloperpwhereby- 'the zein-peptized emulsion- .developedi. Such anemulsion may be used in a mixed grain process for color photography inwhich the couplers are added during development. In the seconddevelopment V another coupler can be introduced and the pH.

emulsion. Y e

If the couplers are dissolved along'with the desired sensitizers in thebenzyl alcohol solution of the zein-peptized emulsion before itisdispersed into the gelatin, removal of the benzyl alcoholby washingwill leave silver halide, sensitizer and coupler in the small dispersedparticles of the zein peptized emulsion. Development in a colordeveloper then develops each particle'as a unit and in View of thehighde'gree of dispersion of such an emulsion, gives colorseparationwith good photographic quality. To obtain good color separation thefollowing factors are necessary: r

(l) Non-diffusibility ofthe sensitizing dye.

(2) Non-diffusibilityof the couplers.

(3 Limited diffusibility of the oxidized-form of the coupling developer.

If desired, three zeinpeptized emulsions may be prepared and dispersedin gelatin, each containing a suitable sensitizing dye and coupler. Theemulsions may then be mixed with. orwithout extra gelatin and coated ina single operation. This procedure would be a simple operation ascompared to the coating of, multi-layer color products.1 i In thepreparing of zein dispersions in accord ance with my invention, thesilver halide is,pre-. pared in a solution of zein in a mixture of .alower alcohol or acetone and water. The dispersion may be preparedeither by runningsolutions of silver: nitrate and alkali metal halideinto the zein solution with stirring or by incorporating one of thereagents in the Zein solution and running 1 ma solution of the otherreagents.

: .Isopropyl alcohol is preferred for the lower ali- 10 raised so as todevelop the zein-peptized patible therewith, such as shellac, rosin,ester resins before dispersing in the gelatin solution. As only a smallquantity of zein is required to peptize the silver halide grain, theadding of extra min to. the emulsion in ,benzyl alcohol modifiesthefs'ize and properties of the dispersed particles. I have found thatconcentrations of zein of the order of -12% are quite satisfactory inthe preparation of silver halide dispersions in which the. amount ofsilver halide may be 2-8 times or more the amount of zein employed. Ihave found that-zein has a good peptizing action on silver halide,and myinvention is not limited as to concentrations or proportions. v Thefollowing examples illustrate my inven tionz.

Example 1.6% g. of zein were dissolved in a mixture of 225 cc. of waterand 250 cc. of isopropyl alcohol, and .55 g. of potassium iodide and37.25 tg. of potassium bromide were dissolved therein. Another solutionwas prepared by dissolving 48.5 g. of silver nitrate in a mixture of 225cc. of water and 250 cc. of isopropyl alcohol. The

. zein solution was raised to a temperature of 57 C.

The silver nitrate solution was allowed to run into the zein solution inseven minutes with good stirring, and the emulsion was stirred for tenminutes at 57 C. The emulsion was then cooled to C. and poured intowater. The mass precipitated and was washed in cold water. The emulsionwas then stirred with 500 cc. of acetone which removed the water fromthe emulsion. 100 cc. of benz'yl alcohol were added. The .emulsion washeated to C. and stirred for fifteen minutes. 35

100 cc. of a20% solution of zein were added, and the emulsion dispersedin' a colloid mill with 300 cc. of a 10% solution of gelatin. Theemulsion was allowed to set by chilling, was noodled, and

It was then melted and coated onto film base. It Was fou'nd that theemulsion readily developed in normal developers and fixed in normalfixing baths, A comparison of the development characteristics of thisemulsion at several different pI-Is with a similar silver bromideemulsion prepared in gelatin is as follows DeveloplO/z T ment pH i 'yTime speed F. -Mz'n. 1 Zein emulsion 30 8.0 0 0 No development.

Gelatin emulsion 70 30 8.0 319 0. 80, Partial development. 2 'Zeinemulsion 70 9 8.6 0 0 N 0 development.

Gelatin emulsion 70 9 8.0 5.0 0.85 Partial development. 3 Zeinemulsion70 6 9.5 1. l 0.27 Partial development. Gelatinemulsion 70 6 9.5 4.252.6 Maximum development. 4 Zein emulsion 70 '2 10.8 3.8 0. 64 Maximumdevelopment. Gelatin emulsionnnn 70 2 10.8 5.5 L43 Partial development.

phatic alcohol as theam'ount ofwater which can be-tolerated isapproximately 50%, this being an advantage in dissolving the silver:nitrate and metal ha1ides;'-Other. lower, alcohols, such as ethyl ormethyLor acetone, may beemployed if desired, it being desirable that themaximum water which will not render the zein insoluble be employedordinarily. Instead of incorporating the silver halide dispersion ingelatin, it may be desired to incorporate it in a solution of afarm Itwas found that the image quality of this emul sion was very good, thedeveloped graininess was low, and the emulsion was free of fog.

Example 2.A'small amount of a solutionof the sensitizing .dye2,3-diethyl-4-methyloxathiazolo carbocyanine iodide was added to asample of the emulsion of Example 1. Exposure behind filters indicatedthat the emulsion had acquired almost no color sensitivity. in thegreen, indicating that the silver halide protected by the zein wasresistant to sensitizing by this dye in the external phase- V Example3.-- Part. of the emulsion of Example 1 was mixed with a solution of thesensitizing dye, 3-ethyl-5-(2-ethyl 1-,benzothiazylidene-isopropylidene)-2-thio-'2,4 (3,5) -oxazoledione, thedye being dissolved in benzyl alcohol and added to a benzyl alcoholsolution of the emulsion before its dispersal in gelatin solution. Theemulsion was then added to gelatin solution, the mass was chilled,noodled, thoroughly washed with water, melted and. coated on film base.This emulsion showed satisfactory sensitivity in the green, indicatingthat sensitization of this type of emulsion can be carried outsatisfactorily if the sensitizing is done before dispersing in gelatinsolution.

Example 4.-Part of the emulsion of Example 1 in its final form(dispersed in gelatin and washed free of benzyl alcohol) was mixed witha silver chloride emulsion dispersed in gelatin. The mixed grainemulsion thus obtained was coated out on film base and developed with11-16, with the pH adjusted to 8.6. In a time of nine minutes gooddevelopment of the chloride emulsion was obtained but no develops-lentof the zein peptized emulsion occurred.

Example 5.Part of the emulsion of Example 1 was coated on plates anddried down without washing out benzyl alcohol. This emulsion processedreadily but it was found that the develop ment restraint at low p'Hswhich is typical of the washed emulsion was absent in this unwashedemulsion. This indicates that removal of the benzyl alcohol is desirablefor protection of the silver halide grain against penetration bydeveloper having low pH.

Example 6.-An emulsion was prepared as in Example 1 but when theemulsion was dissolved in benzyl alcohol no additional zein was added tothe grains. The zein emulsion was dispersed in gelatin solution,chilled, noodled, washed, melted and coated onto film base. It was foundthat the development restraint of the emulsion when subjected todevelopers having a low pH was inferior to that obtained in the emulsioncontaining extra zein. This indicates that the development resistance ofan emulsion can be regulated by controlling the amount of zein used inthe emulsion.

I claim:

1. A method of preparing an optically sensitized emulsion whichcomprises forming zeinpeptized silver halide grains, dispersing thezeinsilver halide mass in benzyl alcohol, adding optical sensitizerthereto and incorporating the dispersion into aqueous gelatin,

2. The product of the process of claim 1.

WESLEY G. LOWE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number

1. A METHOD OF PREPARING AN OPTICALLY SENSITIZED EMULSION WHICHCOMPRISES FORMING ZEINPEPTIZED SILVER HALIDE GRAINS, DISPERSING THEZEINSILVER HALIDE MASS IN BENZYL ALCOHOL, ADDING OTPICAL SENSITIZERTHERETO AND INCORPORATING THE DISPERSION INTO AQUEOUS GELATIN.